Weskan shale member of Pierre shale. Upper 80 feet ["Upper Weskan"] consists of gray clayey shale with few thin beds of bentonite; large tough limestone concretions; rusty cone-in-cone lenses, and thin streaks of concretionary limonite. Lower 90 feet ["Lower Weskan"] consists of gray clayey shale with comparatively abundant beds of bentonite; large limestone concretions common; here and there thin streaks of purple-brown limonite. Underlies Lake Creek shale member of Pierre shale and overlies Sharon Springs shale, basal member of the Pierre. Age is Late Cretaceous.
Type locality: 5 mi north of town of Weskan, [Weskan 15-min quadrangle], Wallace Co., northwestern KS. Type locality of "Upper Weskan" is on a small creek north of Swisegood ranch, in SE/4 sec. 2, T. 13 S., R. 42 W., and best exposure of "Lower Weskan" is on south side of Goose Creek, in SW/4 sec. 4, T. 13 S., R. 40 W., [Weskan 15-min quadrangle].

Source: US geologic names lexicon (USGS Bull. 896, p. 2303).